(1) Field of the Invention
The invention relates to a method for regeneration of photolithographic alignment marks under tungsten layers used in the fabrication of semiconductor integrated circuits, and more particularly to a method for regenerating alignment marks which is retained after such tungsten layers have been smoothed by the practice of chemical-mechanical polishing, which tends to eliminate such alignment marks. This practice allows subsequent precision alignment of photomask patterns required for successful circuit fabrication.
(2) Description of the Prior Art
In the fabrication of semiconductor integrated circuits, patterns of the desired configurations of conductors, insulators, and various and sundry other components of the circuit are formed by photolithographic techniques. These techniques employ stencils, known as photomasks, of the patterns which are transferred in a particular order to the circuit under manufacture. A typical integrated circuit may require that as many as a dozen or more such photomasks be employed in its manufacture. It is necessary that the successive photomask pattern applications be accurately aligned to the previous patterns already laid down on the integrated circuit substrate. For the purpose of such alignments of the patterns, a series of alignment marks are provided in each photomask to allow successive photomask patterns to be conveniently and accurately aligned to already-formed circuit patterns present. As successive layers are built up, it may become more difficult to locate and use alignment marks already present in the circuit patterns, due to the presence of intervening thicknesses of layers on the substrate surface. Additionally, the alignment marks present may be affected by subsequent processing of the pattern layer in such a fashion as to be deleterious to their function by rendering them more difficult to be observed clearly and sharply, or by obliterating them altogether. Thus, the alignment capability available for the metallization layer used for via interconnection such as, for example, tungsten relies on the replication of the sharply-defined edges of an alignment mark produced by the etching of a trench in the silicon oxide layer during the etching of contact via holes in the oxide. In this case, the photomask pattern used for the via hole etch pattern also includes the pattern for the oxide alignment mark trench pattern. The deposition of the tungsten follows the etching of the patterns in the oxide. In the conventional processing of integrated circuits, the deposition of tungsten is followed by a chemical-mechanical planarizing of the surface. This process results in a smooth planar surface which eliminates the sharply-defined edges of the alignment marks and renders subsequent pattern alignment difficult if not impossible. In order to restore a means of subsequent alignment of photomask patterns, additional photolithographic steps involving additional photomasking operations, processing, or both would be required. For example, a method of Cyrix involves removing the smoothed tungsten selectively from the alignment mark area by etching back the tungsten chemically. Both an additional photolithographic step and a chemical etch step are required. Another method employed by Advanced Micro Devices involves pre-alignment prior to tungsten metal deposition in a dedicated system with the possibility of generation of particles. Other solutions are covered in various references such as U.S. Pat. No. 5,401,691 to Caldwell, which teaches planarized alignment marks but does not cover chemical-mechanical polishing of metal layers, and in U.S. Pat. No. 5,369,050 (to Kawai), U.S. Pat. No. 4,992,394 (to Kostelak), U.S. Pat. No. 5,157,003 (to Tsuji), and U.S. Pat No. 5,314,837 (to Barber), which all show alignment marks formation, but not by chemical-mechanical polishing. Likewise, the regeneration of previous alignment marks after chemical-mechanical polishing of tungsten layers is not specifically addressed.